1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically to a passivation structure and its method of fabrication.
2. Discussion of Related Art
Integrated circuits are made up of literally millions of individual devices such as transistors and capacitors formed on a semiconductor substrate. The devices are integrated together by alternating conductive and insulating layers to form functional circuits such as microprocessors. The final layer deposited is typically a passivation layer which is an insulating layer which provides protection against mechanical and chemical damage during assembly and packaging.
An example of a conventional passivation structure is shown in FIG. 1. FIG. 1 shows a substrate 100 having formed on its outer surface a metal interconnect layer 102 which includes a bond pad 104 and interconnects 106. A passivation layer 108 which includes a silicon nitride layer 110 and a thick polyimide layer 112 is formed over metal layer 102 as shown in FIG. 1. A contact opening 114 is then formed through the silicon nitride layer 110 and the polyimide layer 112 to enable an electrical contact, such as a wire bond 116, to be made to bond pad 104 to enable the inputting and outputting of external signals to the substrate. Silicon nitride layer 110, which is a hermetic layer, is formed in direct contact with the lower metal layer 102 to ensure that no moisture path exists to the underlying substrate, especially in the bond pad openings 114.
Although such a passivation structure provides an excellent hermetic seal of substrate 100, device performance suffers due to high metal line-to-metal-line capacitance. That is, because silicon nitride layer 110 has a high dielectric constant (approximately 7.0) and because it is formed in gaps 118 between adjacent metal features 104 and 106, line-to-line capacitive coupling is increased and device performance reduced. Another problem associated with the passivation structure shown in FIG. 1 is that it is difficult to deposit a silicon nitride layer into high aspect ratio gaps 118 to a thickness necessary to obtain a sufficient hermetic seal.
Thus what is desired is a passivation structure and methodology which forms a hermetic seal and which provides low interconnect capacitance.